The present invention relates to a method and apparatus of controlling the positioning of a corrugated board sheet in a processing machine. More particularly, this invention relates to a control method and apparatus which positions various types of processing means, such as a creaser blade and a slotter knife, in a corrugated board sheet processing machine to optimal positions. Fine adjustment (inching manipulation) is performed to set each processing means to the optimal position where a slight error is inevitable due to a peculiarity specific to the corrugated board sheet processing machine despite the processing means being moved to a target value at the time of positioning the processing means.
In the paper industry, a corrugated board sheet processing machine (box making machine) cuts a long corrugated board sheet down to a fixed size and then performs various processes such as printing, creasing and slotting on the sheets cut to size to prepare blanks. Gluing and folding the flap portions of the blanks to yield folded corrugated board boxes is then performed. The box making machine is designed as a combined device having various units arranged in a line including a multi-color printing unit for conducting multi-color printing on sheets of a fixed size, a creaser/slotter unit for making vertical marking on the sheets and slotting the sheets, a hand-hole unit for punching hand holes or the like out of the sheets, and a folder gluer for folding and gluing the sheets. In a corrugator for bonding long webs together to produce a corrugated board sheet, a rotary cutter, for cutting the corrugated board sheet down to a fixed size, is often provided with a slitter for cutting the corrugated board sheet in the vertical direction and a scorer for making horizontal marking. The box making machine generally includes such a slitter and scorer unit.
Each of the units constituting the box making machine is equipped with various types of processing means (e.g., a printing roll and a creaser blade) which directly perform processing such as printing, marking and slotting on arriving corrugated board sheets of a fixed size. The processing means are systematically controlled by a general-control computer system. In accordance with the processing purposes, the processing means are categorized into (1) a type which should be positioned to a fixed position in the rotational direction and (2) a type which should be moved in the axial direction to be positioned to a fixed position. For example, in the printing unit, the plate cylinder should be positioned to a fixed position in the rotational direction and the creaser blade should be positioned in the axial direction of the rotary shaft in accordance with the width of sheets of a fixed size as work pieces. Further, the slotter knife is positioned in the axial direction in accordance with the width of the sheets of a fixed size and is also positioned in the rotational direction to determine the slot length.
The box making machine positions the various types of processing means, such as the creaser blade and the slotter knife, arbitrarily in the rotational direction and axial direction, prior to processing the corrugated board sheet. It is important to improve the positioning accuracy to yield high-quality products. The positioning accuracy that is achieved depends on the following three factors. The precision of positioning the processing means is influenced greatly by (1) the type of the positioning method which is employed in the control system for the box making machine, (2) the mechanical precision of the individual units in the box making machine, and (3) the expansion for computing the specifications of corrugated board boxes to be manufactured. For instance, however high the mechanical precisions of the processing units are or however accurate the expansion of corrugated board boxes is, good positioning accuracy cannot be secured if the positioning method employed in the control system is inadequate. Further, when the expansion of corrugated board boxes is inaccurate, good positioning accuracy cannot be secured even if the other two factors are satisfactory. It is therefore very important to balance out those three factors well in each unit in the box making machine.
The individual units in the existing box making machines manufactured in Japan are supported by excellent manufacturing techniques and have considerably high mechanical precision. However, expansion and contraction caused by a temperature difference, error originating from backlash of a gear system or the like, or error said to be a "peculiarity" of each machine are inevitably present, and there is a limit to eliminating those errors completely. Several proposals have therefore been made on the positioning method of the control system. For instance, a closed loop system may be employed in which a position sensor, such as a linear scale, is attached to movable processing means as a control target, and position information acquired in real time from this sensor is fed back to a servo controller. In this system, a target value based on a control instruction always matches with the current value after positioning, thus ensuring a very high positioning accuracy. The closed loop system has a mechanical system such as a processing means incorporated in the servo loop. However, the mechanical rigidity of such a unit in the box making machine greatly affects the stability of the servo system. A sufficient mechanical rigidity is required which increases the manufacturing cost. Furthermore, corrugated board sheet processing machines do not generally need such high precision to require the use of the costly closed loop system.
There has been proposed a control method disclosed in Japanese Unexamined Patent Publication No. sho 60-40238 for positioning the individual units of a box making machine. The method is a "position feedback system" which uses a current value, acquired by a correction at a time of a previous positioning operation, as a target value in the next positioning operation. The control method employed in, for example, the aforementioned closed loop system that ensures highly-accurate positioning, positions the processing means as specified By the target value or to the corrected value when the next positioning takes place. In this respect, the control method is very effective.
Semi-closed loop systems detect the position, speed, etc. from driving means (motors) of processing means and feed the information back to the control system to provide a significantly high degree of precision control. Such a semi-closed loop system is not inferior to the costly closed loop system. There is also a simple type semi-closed loop system having fewer components than the above systems at the expense of a small reduction in control precision in order to reduce the cost. The simple type semi-closed loop system puts the actual current value in an allowable range of the target value and achieves control without a large variation. The system is favorably employed to control a box making machine unit in a corrugated board sheet processing machine. However, when the simple type semi-closed loop system is used as the basic control system for the box making machine unit, the processing units will not always be positioned as indicated by target values even when under the control of the positioning control system described in the aforementioned Japanese publication. Where a simple type semi-closed loop system has an overrun value of about 1.0 mm at the time of positioning, where positioning is attempted to a target value of 100.0 mm and stops at the position of 101.0 mm, trial processing of a corrugated board sheet is performed right away. If the processed size of the sheet exceeds the allowable range, manual inching may be performed to move the processing means to the optimal value (100.0 mm) and stop it there.
However, even if the current value (100.0 mm) acquired by correction is set as the target value (100.0 mm) for the next positioning as described in the aforementioned Japanese publication, the processing means is positioned near the improper position (101.0 mm) as long as the box making machine unit positions the processing means over the target value by about 1.0 mm. Assuming that this value "101.0 mm" is still within the allowable range of the sheet processing size, if this current value of 101.0 mm is used as the next, corrected target value (101.0 mm ), the processing means will be positioned to 102 mm (101.0 mm+1.0 mm) next time since the box making machine unit tends to shift the processing means by about 1.0 mm to the plus side. In other words, the error is gradually accumulated so that the processing means will be positioned to a further improper position.